This application claims the priority benefit of Taiwan application Ser. No. 91100098, filed Jan. 7, 2002.
1. Field of the Invention
The present invention relates to a bump fabrication method, and in particular, a method of forming good quality bumps, avoiding etching the solder layer pattern after the exposed wetting layer, barrier layer, and adhesion layer have been removed.
2. Description of the Related Art
The so-called bump fabrication method commonly used in flip chip technology is the formation of Under Ball Metallurgy (UBM) on the external connection point on a wafer. A wafer bump is formed on the under ball metallurgy, and finally, the wafer bump is directly connected to the substrate.
General UBM is used as an interface between the bump and the pad and therefore, the UBM must possess low stress, good adhesion, strong corrosive resistance and excellent wetting properties. Normally, a UBM comprises three layers of metals. Each respectively improves the bonding between the metal and the adhesion layer, improves the wetting layer of the bump, and provides a barrier layer in between two layers. The function of the barrier layer is to prevent a particle of the adhesion layer from penetrating the wetting layer, or to avoid a particle of the wetting layer penetrating the adhesion layer.
In the conventional method of fabricating bumps, after a solder layer is electroplated, the solder layer is used as a mask, and an etching method is used to remove the wetting layer and the barrier layer, and then the adhesion layer. Finally, by employing a re-flow process, the solder layer is formed into a bump. However, in the course of removing the barrier layer, due to the nitric acid contained in the etching solution, the solder layer being used as a mask is also etched when the wetting layer and the barrier layer are removed.
When the solder layer is etched by the etching solution two problems will arise.
(1) If the etching solution etches the surface of the solder layer, the volume of the solder layer is insufficient so that the height difference of the bump after a re-flow process increases, and the object of the external connection of the bump may not be achieved.
(2) When the etching solution etches the bottom of the solder layer, the connection face between the solder layer and the UBM will decrease. This will reduce the bonding force between the solder layer and the UBM, and further, the solder layer may be stripped off.
A conventional method of fabrication bumps is described as follows: Referring to FIGS. 1 to 7, together with FIG. 8, wherein FIGS. 1 to 7 are sectional views showing the conventional method of fabricating bumps, and FIG. 8 is a flowchart block diagram showing a conventional method of fabricating bumps.
As shown in FIG. 1, the conventional method first provides a wafer 100 (S100 of FIG. 8) having a plurality of pads 102 and a passivation layer 104 covering the surface of the pad 102, and exposing the pad 102. On the surface of the wafer 100, an adhesion layer 106, a barrier layer 108, and a wetting layer 110 (S102 of FIG. 8) are formed. As shown in FIG. 2, a patterned photoresist layer 124 is formed to cover the surface (S104 of FIG. 8) of the wetting layer 110. This patterned photoresist layer 124 is provided with a plurality of openings 126 exposing the wetting layer 110.
As shown in FIG. 3, the openings 126 are electroplated with a solder layer 114 (S106 of FIG. 8).
FIG. 4 shows the removal of patterned photoresist layer 124 (S108 of FIG. 8). As shown in FIG. 5, the solder layer 114 is used as a mask to remove the wetting layer 110 and the barrier layer 108 (S110 of FIG. 8). Next, as shown in FIG. 6, the solder layer 114 is used as a mask to remove the adhesion layer 106 (S112 of FIG. 8). Finally, as shown in FIG. 7, a re-flow process (S114 of FIG. 8) is performed, and the solder layer 114 is formed into a bump 116, wherein the steps of S110 and S112 can be combined into a single step.
When the solder layer 114 is used as a mask to remove the wetting layer, the barrier layer and the adhesion layer (as shown in FIG. 6), due to the nitric acid contained in the etching solution, the solder layer will be etched and this will cause an insufficiency with respect to the volume of the solder layer, so that the height difference of the bump after a re-flow process increases, and the bonding force of the solder layer and the UBM is reduced, so that the solder layer may be stripped off.
Accordingly, it is an object of the present invention to provide a bump fabrication method, wherein in the course of etching the barrier layer, the solder layer will not be etched, and the height difference of the bump after a re-flow process is maintained to a limited range, and the bonding force between the bump and the UBM remains excellent, which provides excellent bump quality.
Another object of the present invention is to provide a bump fabrication method, comprising the steps of providing a wafer having a plurality of bump pads and a passivation layer on the surface of the wafer and exposing the bump pads; forming an adhesion layer, a barrier layer and a wetting layer in sequence on the surface of the wafer; defining the wetting layer and the barrier layer to form a plurality of Under Ball Metallurgy (UBM) patterns exposing the adhesion layer, wherein the UBM patterns correspond to each of the bump pads; forming a plurality of solder layer patterns on the surface of the UBM patterns, wherein each of the solder layer patterns corresponds to the UBM patterns; removing the exposed adhesion layer; and performing a re-flow process to form a bump on the solder layer pattern, wherein the wafer is provided with a plurality of pads, each UBM pattern respectively corresponding to each of the pads, and each solder layer pattern respectively corresponding to each of the UBM patterns.
In accordance with an aspect of the present invention, prior to forming the solder layer pattern, the wetting layer and the barrier layer are removed, and after a solder layer pattern is formed, only the exposed adhesion layer is removed, avoiding etching the solder layer pattern in the course of etching the solder layer and the barrier layer, and therefore the volume of the required solder layer pattern can be maintained. Thus, the height of the bump after the re-flow process is maintained at an appropriate range and the required bonding force between the bump and the under ball metallurgy layer pattern can be maintained so as to improve the reliability.